As a semiconductor device manufactured in a way that a trench is formed in a semiconductor substrate including silicon (Si), and then the relevant trench is filled, a semiconductor device having a PN column layer is disclosed, for example, in JP-A-9-266311.
FIG. 7 is a schematic section view of a semiconductor device 100, showing a representative example of a semiconductor device having the same PN column layer as a related art.
The semiconductor device 100 shown in FIG. 7 is an N-channel vertical MOS transistor having a PN column layer 1a, which is formed in a middle portion along a section of a semiconductor substrate 1, and is rectangle in the section of the substrate, and has P-type conduction regions 1p and N-type conduction regions 1n in a striped repetitive pattern. The PN column layer 1a acts as a drift layer of the vertical MOS transistor, and configures a super junction (SJ) structure.
In an on-state of the vertical MOS transistor formed in the semiconductor device 100, an electron flown out from a source region 1s is flown into an N-type conduction region 1n of the PN column layer 1a through a channel formed in a P-type conduction layer 1c in the periphery of a trench gate G, and reaches to a drain region 1d. Therefore, impurity concentration is increased in the N-type conduction region as a drift region 1n of the PN column layer 1a, thereby on-resistance of the vertical MOS transistor formed in the semiconductor device 100 can be decreased. On the other hand, in an off-state, the PN column layer 1a acting as the SJ structure can be perfectly depleted to have high breakdown voltage, i.e., high withstanding voltage. In the same way, conduction types of all components of the N-channel vertical MOS transistor shown in FIG. 7 are reversed, thereby a P-channel vertical MOS transistor having the SJ structure is obtained. The semiconductor device 100 having low on-resistance and high breakdown voltage is structurally featured in having the PN column layer 1a in which the P-type conduction regions 1p and the N-type conduction regions 1n are in the repetitive pattern.
A formation method of the PN column layer 1a as a feature of the semiconductor device 100 is disclosed, for example, in JP-A-2004-273742, which corresponds to U.S. Pat. No. 7,029,977.
According to the method, a trench is formed in a surface portion of a semiconductor substrate in a first conduction type, then a semiconductor in a second conduction type is epitaxially grown in the trench to fill it by low-pressure chemical vapor deposition (LP-CVD), thereby the PN column layer 1a is formed. In such a formation method of the PN column layer using trench formation and subsequent filling, impurity concentration distribution can be made uniform in a depth direction in the trench (thus, thickness direction of the PN column layer 1a) unlike a formation method of the PN column layer using impurity diffusion.
To increase breakdown voltage of the vertical MOS transistor formed in the semiconductor device 100 shown in FIG. 7, the PN column layer 1a as the drift layer needs to be formed thick. For example, to obtain breakdown voltage of 600 V, the PN column layer 1a needs to have thickness equal to or more than 30 μm. Furthermore, to allow the PN column layer 1a, which was formed with high impurity concentration for decreasing on-resistance, to be completely depleted, width of the PN column layer 1a needs to be narrowed to about 1 μm. Therefore, when the PN column layer 1a is formed according to the method, a trench having a large aspect ratio (a ratio between the depth of 30 μm and the width of 1 μm is equal to 30) and filling of the relevant trench are necessary for the semiconductor device 100, which has low on-resistance and high breakdown voltage.
On the other hand, when the semiconductor is epitaxially grown by LP-CVD to fill the trench having the large aspect ratio, the following difficulty is given.
FIGS. 8A to 8B are enlarged section views of a trench 1t formed in the semiconductor substrate 1, showing an aspect of the related art during trench-filling by epitaxial growth of the semiconductor using LP-CVD.
As shown in FIG. 8A, in the trench 1t having a large aspect ratio (i.e., depth d/width w), a silicon (Si) source gas hardly reaches to a bottom of the trench 1t during LP-CVD. Therefore, as shown by size of arrows in the drawing, a growth rate of an epitaxial layer 1e is increased at an upper portion of the trench 1t. As a result, as shown in FIG. 8B, a top (i.e., an opening) of the trench 1t is closed in an early stage, consequently a void 1v that is imperfect filling tends to be formed within the trench 1t. Moreover, since an epitaxial layer 1e is grown from sides of the trench 1t, thereby crystallinity of the epitaxial layer 1e may be deteriorated in the periphery of the void 1p. In particular, when the aspect ratio of the trench 1t is 30 or more, inferior crystals tend to be formed at the void 1v. When the inferior crystals are present in the periphery of the void 1v as shown in FIG. 8B in the PN column layer 1a in FIG. 7 formed by such trench-filling structure, they cause decrease in breakdown voltage of the semiconductor device 100 or inverse leakage current by defective connection.
Accordingly, as described before, as the aspect ratio of the trench is increased to obtain a semiconductor device 100 having lower on-resistance and higher breakdown voltage, trench-filling structure cannot be fabricated, leading to decrease in breakdown voltage of the semiconductor device 100 or inverse leakage current by defective connection due to the inferior crystals at the void 1v. 